The invention relates to a process for the preparation of blocked isocyanates, particularly polyisocyanates, with at least two isocyanate groups, which are blocked with blocking agents containing acid hydrogen. The blocked isocyanates obtained are particularly suitable as cross-linking agents for polyols, particularly in the preparation of lacquers.
The preparation of blocked isocyanates, particularly polyisocyanates, by reaction with blocking agents containing acid hydrogen such as dialkyl malonic esters and alkyl acetoacetic esters is known, for example, from DE-A 30 01 060, DE-A 24 36 872 and DE-A 23 42 603. The known processes are carried out using catalysts which are required for the reaction. Alkali metals, preferably sodium, alkali alcoholares, preferably sodium alcoholates, and alkali phenolares are mentioned for the reaction with malonic diesters. Zinc acetyl acetonate is also described for the reaction with acetoacetic esters.
Said known catalysts are strongly basic and difficult to handle, Moreover, it has become apparent that the reaction products obtained are discoloured, frequently yellowed. The catalysts dissolve in the reaction medium and may be removed only by expensive measures.